In a simple cycle (i.e. single stage) vapour compression cycle refrigeration system, the refrigerant ideally enters the evaporator as a mixture of saturated liquid and saturated vapour. Full utilization of the heat transfer surfaces in the evaporator circuits requires the presence of liquid refrigerant in or on all parts of the tubes that make up the various circuits of the evaporator. In the evaporator the liquid refrigerant changes, under relatively constant pressures, into a vapour and absorbs heat from the zone serviced by the evaporator. The refrigerant then leaves the evaporator preferably as a saturated vapour, or as a slightly superheated vapour.
The refrigerant next enters the compressor, where it is isentropically compressed to the condensers operating pressure. The refrigerant flows from the compressor and through the condenser under fairly constant pressure conditions, and dissipates heat to the atmosphere.
Finally the refrigerant leaves the condenser as a liquid and flows through the expansion valve and back to the evaporator, with part of the refrigerant flashing into vapour as the line pressure drops across the expansion valve.
Such a simplistic system only operates efficiently and safely within a narrow range of ambient heat loads. Since normal seasonal and even diurnal variations in ambient conditions impose loads outside of the range that can be handled by simplistic systems such as that described hereinabove, steps are usually taken in the design of modern refrigeration equipment so as to provide for a broader range of operating conditions.
These steps typically include the use of thermostatic expansion valves in combination with capillary tubes, flat valves and automatic expansion valves. Even so such equipment is capable of dealing only with those variations in heat load that are imposed by a fairly modest range of ambient operating conditions.
Even with such additions, however, ambient load conditions can still exceed design limitations, and further precautions have been found to be necessary. It is important, for example, that no substantial amount of liquid refrigerant be carried out of the evaporator with the vapour that is returned to the compressor. This problem does arise, however, when the refrigeration equipment is operated at ambient heat loads below the lower limits of the designed heat transfer capacity for that equipment. In such circumstances the amount of ambient heat available at a given flow rate of refrigerant through the evaporator is insufficient to vapourize substantially all of the liquid present in the evaporator. The liquid refrigerant that does exit the evaporator must be trapped before it reaches the compressor, or serious loss of compressor lubrication is likely to result. In-line liquid traps ranging from simple "U" tubes, or swan necks, up to complex suction gas/liquid heat exchanger and suction pots are used for this purpose, with the choice of apparatus depending on the anticipated operating loads.
As has already been mentioned, the full utilization of the heat transfer surfaces in all evaporator circuits requires the presence of liquid refrigerant in or on all parts of the tubes that make up the heat transfer surfaces in the various evaporator circuits. Under very low load conditions, the need to maintain sufficient liquid refrigerant in the evaporator, and the proportionately small amount of heat taken up by that refrigerant relative to the design capacity of the system, may result in too large an amount of liquid refrigerant leaving the evaporator and exceeding the capacity of the above-mentioned traps. The consequences of returning liquid refrigerant to the compressor has also already been mentioned.
Other approaches are therefor used in tandem with those mentioned hereinabove. In small systems the compressor is merely shut down when the cooling thermostat setting has been satisfied. In large systems that option is not as readily available, because of the wear that attends the on/off cycling of the large compressors used in these systems.
Accordingly, in large systems employing centrifugal compressors the capacity may be varied to match a change in ambient loading by: (1) varying the speed at which the compressor is driven; (2) adjusting vanes at the inlet to the impellers; (3) throttling the suction gas; or, (4) varying the condenser pressure. Methods 1 and 2 require feedback controls with their attendant increased capital and maintenance costs. Attempting to control the capacity by either throttling the suction gas or varying the condenser pressure results in reduced system efficiency.
In large systems using the more common reciprocating compressors (and in which the lubrication problems are much more serious than with centrifugal compressors), capacity control can be accomplished through several means which are used in combination with one another. The most common approach is to unload the compressor through a series of unloading stages until a final unloading stage, whereupon a controlled refrigerant bypass of the condenser and the expansion valve is employed to reroute hot-gas to the evaporator, by directing gas from the compressor discharge into the low pressure side of the system, at a point either up or downstream of the evaporator. This approach is known to seriously reduce system efficiency since even though the reduced condenser pressures which normally accompany a reduced system load result in a saving in compressor power, it may interfere with the flow of liquid refrigerant through the expansion device and cause unsatisfactory operation of the system. This is because the expansion valve meters less refrigerant to the evaporator when the system is operated at reduced condenser pressures. In a typical installation equipped with such a hot-gas bypass system, the discharge bypass valve will attempt to compensate for the substantial reduction in suction pressure when the compressor is in its final unloading stage and maintain a given predetermined (i.e. design) pressure. With the reduced demand for refrigerant and less volume of liquid throughput, the expanding liquid has less velocity in the evaporator. It has now been found that this allows the hot gas, that has entered the auxiliary side connector upstream of the evaporator and has been merged into the refrigerant flow leaving the expansion valve and entering the distributor, to push the expanding liquid refrigerant away from some of the distributor tubes. This in turn causes an uneven distribution of vapour and liquid within the various evaporator circuits. The desuperheating that then takes place within the evaporator not only renders some circuits inactive for cooling purposes, but actually results in localized heating of the ambient environment over certain portions of the evaporators heat exchange surface.
One example of the type of installation where these problems are particularly acute is in ship-board airconditioning systems. These "mobile" systems must have a design capacity which will deal with large ranges of sensible heat variation, particularly in the case of ocean-going vessels which often traverse both tropical and high latitudes.